Highly reliable, short range communications are a vital necessity for a host of closely coordinated activities that involve high performance equipments and precision technologies as well as those situations that usually call for split second timing. The military is one user where all these factors are acutely present.
The military also requires covertness, security and nonsusceptibility to being jammed. Since centralized command posts are targets for hostile action and the reliable communication links are a vulnerable component of command, control, and cummunication networks, higher reliability communications are vitally needed. Many times, communication silence is imposed, since otherwise, a hostile force can readily locate friendly locations by directionally homing in on a transmitter. In this regard, active sensors such as radar, although excellent in the surveillance problem, are directionally detectable like a conventional radio transmitter; while passive sensors are less vulnerable, they do not provide accurate information unless they are internetted with a high bandwidth communication network.
Short wavelength signals, in the millimeter range, have proven quite acceptable for line-of-sight communications for they tend to deny the information to a hostile monitoring station; yet, their usefulness is limited, particularly in hilly terrain. Still shorter wave lengths within the optical and infrared regions rely on lasers which offer very tight, highly collimated beams. Receivers with a narrow field-of-view limit the noise from daylight and other background radiation sources so that reliability is quite acceptable; however, once more, elaborate pointing and aiming efforts are required for the receivers and transmitters to pass the beams and uneven terrain limits their usefulness.
One hitherto unexploited spectrum for communications lie in the optical spectrum and is identified as the "solar blind" region This is a specific spectral region from 230 to 280 nanometers in which the ozone layer surrounding the earth absorbs the sun's radiation and, hence the expression "solar blind". A detector that is only sensitive to this spectral region has the capability to operate in the daylight, even while pointing at the sun, and pick up little background radiation because the UV spectrum from 230 to 280 nanometers is absorbed by the ozone layer. Thus, a detector operating in this wavelength region need not be directional and will have an increased performance by orders of magnitude because of the reduction of the background noise. Furthermore, precise alignment of the transmitter and receiver is dispensed with since a detector does not have to operate in the line-of-sight but can function in a wide field-of-view mode to sense radiation scattered by the modulated UV signal.
A wide field-of-view receiver could be assembled that has a detector that is sensitive to the scattered radiation from a transmitter to increase the receiver signal's strength. Calculations using a single scatter model of radiation from atmospheric aerosols show that the ultraviolet solar blind detector can be out of the direct path of the transmitted beam, and when exposed to the scattered beam, it picks up sufficient radiation for system operation. This provides a non-line-of-sight operational capability. The calculation shows that as the divergence of the transmitter beam is increased, performance degrades somewhat but not too seriously. Thus, an incoherent source that is only somewhat directional will suitably function as a transmitter; since such a source can be employed, a bid advantage is apparent as almost all lasers and certainly all ultraviolet lasers are inefficient and do not emit watts of average power. On the other hand, an incoherent UV source emits average power of watts efficiently with uncomplicated technology. There are no ultraviolet lasers that can compete in this regard.
It might be expected that UV lasers could be adapted to a UV communication system. At first glance, a laser operating in the "solar blind" region could zero in on a line-of-sight or in a scattering mode of transmission. But, when the power inefficiencies and relatively delicate and bulky structures of the UV lasers are considered, they are generally unsuitable for use in the field. For example, a pulsed xenon lamp has efficiencies of conversion of electrical power into usuable ultraviolet radiation of about 0.2% with average output powers less than 0.1 watt. Such a low power projection must necessarily be accurately aligned to a field-of-view receiver and has a greatly limited usable range. When pulse modulation was tried, the xenon lamp could only be modulated at lower frequencies.
Thus, there is a continuing need in the state-of-the-art for a secure and covert communications system operating in the "solar blind" ultraviolet spectrum having a transmitted power of multi-watts to assure reliable non-line-of-sight communications.